Remove windowpanes if broken. If window panes are not broken, lower or raise the window sash to permit installation of carriage bolts described below.
Neatly cut a single piece of 1/2" CD exterior grade plywood that is sized to fit snugly inside the window opening against the windows stop. Consider installing 2" x 4" blocking within the opening for back of plywood to rest against snugly.
Cut an opening centered and 6" below the top of the plywood and install a metal soffit vent that covers the opening but also allows light to enter the structure once the plywood has been installed. Use screws to attach the vent to the plywood.
Cut at least two 2" x 4" wood support members that are 16" wider than the window opening. The support members are to be mounted horizontally and flush against the interior window casing with 8" extending left and right of the window opening. The top support should be located within 1/4 and 1/3 of the window opening's height from the top of the opening. The bottom support should be located within 1/2 to 1/3 of the window opening height from the bottom of the opening.
Drill at least two holes in each of the 2" x 4" support members and then drill holes in the plywood that line up with the holes drilled in the support members.
Insert a washer over the end of a 3/8" diameter round, smooth head, carriage bolt.
Line up each hole in the 2" x 4" support member and the plywood. At each hole, insert a 3/8" diameter carriage bolt – from exterior to interior – through the plywood, through the window opening, and through the 2" x 4" and plywood toward each other until there is no play.
Prime the exterior surface of the plywood.
On the exterior, caulk the perimeter edges of the plywood.
Paint the exterior surface of the plywood a neutral color or color similar to the window.